Temperature dependent structural integrity of S2-glass and E-glass fiber reinforced epoxy laminates under indentation and flexural loading

Glass fiber reinforced polymer composites are extensively utilized in aerospace, automotive, and marine structures, where they frequently encounter broad thermal fluctuations. Despite their widespread use, the fundamental differences in how distinct glass fiber types interact with polymer matrices under variable temperatures remain underexplored. This study presents a comprehensive, comparative investigation into the temperature-dependent mechanical degradation mechanisms and damage tolerance of S2-glass/epoxy and E-glass/epoxy laminates fabricated via vacuum infusion. Quasi-static indentation (QSI) and three-point bending tests were performed at sub-ambient (−25°C), room (25°C), and elevated (75°C) temperatures, all situated below the measured glass transition temperature (Tg = 110°C). The macroscopic structural response was quantitatively correlated with microscopic failure mechanisms elucidated through scanning electron microscopy (SEM) and validated by statistical analysis (ANOVA). S2-glass composites exhibited superior structural integrity and thermal stability across the entire temperature spectrum. At sub-ambient conditions, restricted polymer chain mobility led to increased stiffness but reduced ductility, promoting brittle matrix cracking. Conversely, at elevated temperatures, increased free volume induced matrix softening and relaxation of compressive residual stresses at the fiber-matrix interface, resulting in a transition to ductile, interface-dominated failure. The findings establish that the intrinsic chemical composition and enhanced interfacial adhesion of S2-glass significantly mitigate thermo-mechanical degradation, providing critical design guidelines for ensuring the structural reliability of composite components in thermally demanding environments.